The tunnels will allow the T Third Line trains to travel quickly beneath SoMa, Union Square and Chinatown when the Central Subway opens, cutting travel times by more than half along this busy corridor.

Like Mom Chung, Big Alma is 350 feet long and weighs 750 tons. In the coming months, the two machines will travel north under 4th Street, Stockton Street and Columbus Avenue, excavating and constructing San Francisco's first new subway line in decades.

The two machines will keep some distance between them as they move forward. Currently they are about 1,800 feet apart, with Big Alma under 4th and Harrison and Mom Chung near 4th and Mission.

"Today we are another big step closer to building the 21st Century transportation system our world-class city needs and deserves," said San Francisco Mayor Edwin Lee. "The Central Subway and the tunnels we are excavating today are essential to our vision and crucial to building and maintaining a reliable, modern public transportation system for San Francisco residents and visitors."

The TBMs will excavate and construct the 1.5-mile-long tunnels at an average pace of 40 feet per day, though their pace will vary based on ground conditions and other factors. Big Alma will move more slowly during the first 500 feet of tunneling, as Central Subway crews test the TBM and calibrate its many functions.

Currently Mom Chung is mining 20 hours a day, five days a week, with maintenance on Saturdays and between mining shifts. Most of the TBMs' journey will be through two major ground formations: the Franciscan complex, a bedrock formation that forms Nob Hill and the Colma formation, a dense mixture of sand and clay. The TBMs travel so far beneath the surface, between 40 and 120 feet underground, that no vibration or noise will be felt above ground when they pass below.

To launch, Big Alma pushed off of a steel frame as her cutter head began to spin. As Big Alma tunnels, the machine will stop every five feet to install the concrete segments that make up the tunnel's lining. The concrete segments are installed within the back of the TBM's cylindrical shield. The machine lifts the segments into place and then crews bolt them together. Hydraulic jacks within the shield then push off of the newly installed tunnel lining, propelling the massive machine forward. A crew of about 10 people operates each machine and bolts concrete tunnel segments together. The Central Subway is expected to open to the public in 2019.